Use npm-installed version of phantomjs.

Versions newer than 1.9.2 appear to have font problems, so we set an
upper bound on the version for now.
Add pandoc pagebreak filter
Add a custom PDF footer with phantomjs
Add some extra html markup via javascript.

In order to style up some elements for pdf generation, it’s necessary
to add some spans via javascript. Some of this can be fixed by
modifying lesson markdown / manifests, but it’s perhaps simplest to
do it this way.
Add a legend template; move it into position in js
Lots of new print styles!
Rejig rasterize-related files a bit